^I think you can only get onto memorial drive if you're on the other side of the Red Line tracks. And the Jersey barriers kind of preclude using the other set of ramps or turning around.
Wadsworth St is right there, I would imagine people saw the traffic before the point of no return. Although I'm surprised they didn't leave the ramps open, unless they needed the space as a staging area. Granted, you can't turn onto the Harvard Bridge, and a zillion people using that u-turn at Ames St could get dicey.
BussesAin'tTrains, yes, I am convinced that the majority of drivers know literally only one path from A to B. Then again, I can't understand why anyone would willingly drive through Charles Circle to begin with. Or really any part of the city east of Mass Ave. I would rather a spike through the head.